In known helicopter rotor assemblies, and particularly rotor assembly 100, cyclic and collective pitch are normally controlled by an oscillating-plate control device 101 substantially comprising two concentric rings 102, 103. The outer ring 102, i.e. the one radially outermost with respect to the central axis A, is supported for rotation, on bearings 104, by the inner ring 103, and is connected angularly to a drive shaft 105 to rotate a number of pitch-change rods 107, each hinged eccentrically to a respective blade (not shown in FIG. 8).
The inner ring 103 is fitted coaxially to a substantially sleeve-like movable member 110, in turn mounted to slide axially on a fixed pylon 108, through which the drive shaft 105 extends in axially-fixed, rotary manner.
Movable member 110 is bounded by a spherical annular outer surface 111, which engages a spherical seat 112 on inner ring 103; and inner ring 103 is normally locked angularly to spherical annular outer surface 111 of movable member 110 to oscillate in any direction with respect to movable member 110 about the centre of outer surface 111.
Control device 101 also comprises a number of hydraulic actuators 113 equally spaced about and extending substantially parallel to drive shaft 105, and which exert thrust on inner ring 103 to jointly move inner ring 103, outer ring 102 and movable member 110 axially with respect to pylon 108, and to oscillate rings 102 and 103 with respect to movable member 110 about axes substantially perpendicular to axis A of drive shaft 105.
Movable member 110 is connected to pylon 108 by a compass-type connecting device 114 for preventing rotation of movable member 110 about axis A with respect to pylon 108.
More specifically, connecting device 114 substantially comprises a bracket 116 projecting outwards from pylon 108; a first connecting rod 117 hinged at one end to bracket 116; and a second connecting rod 119 hinged at one end to movable member 110, and at the other end to the end of connecting rod 117 opposite the end connected to bracket 116.
Connecting device 114, pylon 108, and movable member 110 together define a constraint assembly 140 of helicopter rotor assembly 100.
Other examples of known helicopter compass connecting devices are illustrated in EP-A-1031509 and FR 2,771,708.
Though functionally valid, compass connecting devices such as 114 are fairly complex (comprising three hinged parts), are relatively heavy and expensive, and call for constant lubrication and inspection, by being subjected in use to alternating loads which tend to increase slack between connecting rods 117 and 119.
Moreover, to conform with aircraft design redundancy requirements, the number of such connecting devices must be doubled, thus further compounding the above drawbacks.